1. Field of the Invention
The present invention relates to an image forming apparatus that performs gradation correction control by forming and detecting rows of area coverage modulation patterns having patterns of different gradation values in steps by area coverage modulation method.
2. Description of the Related Art
In electrophotographic image forming apparatuses, gradation is expressed by area coverage modulation method for expressing pseudo-density by varying an area ratio of toner adhering region. Most of image forming apparatuses have several pattern types to match the image quality desired by the user and selectively use them for different types of output images. For example, dot-type patterns are used to express strings of characters, while line-type patterns in which a phenomenon such as a moire is not likely to appear are used to express gradation for photographs and the like for which more importance is placed on the gradation such as shades of colors. These pattern types have different gradation characteristics. Accordingly, when the gradation characteristics are changed by changes in operating environment such as temperature and humidity and changes of each image forming unit over time, it becomes necessary to correct the gradation characteristics for each of the pattern types.
Conventionally, for the gradation correction of each pattern type, a gradation correction method by reading density information of rows of area coverage modulation patterns formed on a paper sheet with a scanner and by feeding it back to an image forming unit to correct the gradation characteristics has been used. However, because it requires manual operation of reading the rows of area coverage modulation patterns formed on the paper with the scanner, when to perform gradation correction is left to a user or service personnel to decide. In addition, such operation of gradation correction is cumbersome for users. Consequently, a gradation correction method that automatically performs the gradation correction and stabilizes gradation characteristics is desired.
Japanese Patent Application Laid-open No. 2003-270868 discloses that gradation characteristics are corrected based on density information by receiving the information of pattern type set in a print controlling unit and forming patches of the same pattern type for gradation correction. However, when image signals of different pattern types are frequently received, it is not possible to correct the gradation characteristics for all the pattern types and thus, the difference in image quality may become large depending on the pattern types. The disclosure describes that when the pattern type is changed, the gradation correction is performed in response to the pattern type changed. In this method, the correction is performed every time the pattern type is changed, thereby increasing the waiting time of the user.
Japanese Patent Application Laid-open No. 2004-88257 discloses that the number of patterns for rows of area coverage modulation patterns to be formed in an image forming apparatus is changed based on pattern types. For example, a smaller number of patterns is used for the pattern types applied to font objects that are relatively less critical to shades of colors, while the number of patterns that obtains correction accuracy comparable to conventional ones is used for the pattern types applied to the objects for which more importance is placed on shades of colors. Generally, the larger the number of patterns for gradation correction is, the more accurate gradation characteristics are obtained. However, to shorten the time required for the correction, a method is used to approximate the gradation characteristics from the detection result of a smaller number of patterns. For this reason, in the disclosed method, the approximated gradation characteristics changes depending on the number of patterns and thus, the gradation stability may not be maintained.
Furthermore, because the gradation correction is performed by forming a plurality of pattern types, the time which takes for the gradation correction becomes longer, thereby increasing the waiting time of the user.
Japanese Patent Application Laid-open No. 2004-64358 discloses that gradation correction is performed by forming on an image carrier rows of area coverage modulation patterns of two or more pattern types for gradation correction control and by selecting a gradation table to correct depending on an operating condition of the apparatus. The operating condition here in the disclosure is triggered at start-up of the apparatus, for every predetermined number of printouts, and the like. In this method, however, the pattern type that the user is to use and the pattern type to which the gradation correction is performed may differ and thus, the effect of gradation correction for the desired pattern type may not be obtained. Additionally, the rows of area coverage modulation patterns of two or more pattern types are formed on the image carrier to detect density information, which leads to a problem in that the time required for the gradation correction is long.
While image forming apparatuses of recent years are required to provide output images of high quality with a bare minimum of image quality adjustment time, in general, the image quality adjustment time and the image quality are in the relation of trade off. Because the user cannot perform any printing during the operation of image quality adjustment, it is preferable that the time required for image quality adjustment be shorter. On the other hand, the demand for image quality is ever increasing. Accordingly, it is inevitable to appropriately correct various characteristics relating to image quality such as gradation characteristics to maintain gradation stability.